The main principle of Light Emitting Diodes (LEDs) is by applying current to light emitting materials to achieve light emitting effect. In recent years, high-power LEDs bring LED light source applications into a whole new era. High-power LEDs are characterized by small size, high luminance of light and maximum light output per unit area, which eliminate the use of traditional LED arrays. Accordingly, the LED light source may provide better light source characteristics; and facilitate optical designs and applications. In the meantime, the small size and high luminance of the high-power LEDs open a new door for the LED light sources and also allow design of LED applications to be more flexible. Common applications include car lights, flash lights and other lighting applications with very small size and high power, as well as display applications such as LED backlight modules, project light sources, and outdoor displays. Moreover, current LED light-emitting efficiency is only 15% (external quantum efficiency), so most of the input energy converts to heat, which results in temperature rise. As shown in FIG. 1, the operating life of an LED that operates under high efficiency reduces with increase in temperature. The device may even fail due to overheating. Thus, heat dissipation needs to be taken into consideration in LED packaging, so as to allow the chip in the LED package to withstand more power, thereby increasing its luminance.
Generally, there are three ways of heat dissipation in LED packaging: conduction, convection and radiation, wherein the effect of radiation in dissipating heat is relatively limited. In a traditional LED heat dissipating system, the heat of an LED chip will first conduct to the external environment through the package. In order to improve the heat dissipation of a high-power LED package in the prior art, increasing the thermal conductivity of the conducting path is the main area of focus. As for the heat dissipating design for heat convection, metal fins or larger area of metal substrates are usually added to the whole system, so as to increase the surface area contacting the air. There is a large amount of related patents, for example, U.S. Pat. No. 6,274,924, No. 6,830,496, No. 6,739,047, No. 6,637,921, No. 6,084,252, No. 6,705,393 B1, and Taiwan Patent No. 213446 and 220351.
As an example, the U.S. Pat. No. 6,274,924 proposes a surface mountable LED package. As shown in FIG. 2, this patent is directed to adhering an LED chip 21 on a metal slug 23 for heat dissipation, such that heat can be conducted to the bottom of the package due to high thermal conductivity of the metal slug 23.
However, the contact area of the metal slug 23 and the air is limited. If the application is to meet the product standard, the package has to be further adhered to a metal fin, a metal substrate or other material with high thermal conductivity. However, such materials usually have a larger size. As a result, more spaces are occupied by such larger-area element or material with high thermal conductivity for increasing the surface area with area, which hinders the application of LEDs to small-size application. Meanwhile, since this conventional technique has to be connected to larger-sized heat dissipating substrate or using metal heat dissipating substrate or fin for heat dissipation. Thus, additional cost is required for circuit fabrication.
U.S. Pat. No. 6,739,047 discloses a method for forming a high-power device module. As shown in FIG. 4, module 6 employs a combining structure of a metal substrate 61 and a ceramic 63, both of which are good heat dissipating materials, to allow heat generated during operation of a chip 65 to be conducted via the surfaces of those two to air. The module is electrically connected to a motherboard 60. However, similar to the last patent, there is no design that increases the area that contacts the air.
U.S. Pat. No. 6,637,921 proposes a packaging system with swappable light sources. As shown in FIG. 5, the system is provided with a base plate 81 made of metal as the heat dissipating element and also support for LED light source 83. A plurality of denting heat dissipating structure is disposed on the rim of this system (i.e. the peripheral of the base plate 81) as heat dissipating fins to enhance convection of heat. However, such heat dissipating design is at the rim of the whole system, it also occupies a relatively large area and is difficult to apply to small-size products.
U.S. Pat. No. 6,084,252 teaches a semiconducting light emitting device that adds a base design with a special shape for heat dissipation to the package base of a conventional LED lamp. As shown in FIG. 6, the heat dissipating element 6 of this patent is provided with a plurality of curved indentations 101, which is one of a few that allows heat airflow to be guided into the LED package body. Although the curved indentations 101 of this patent allows the surface area contacting the air to be increased, but the location of the LED chip does not provide direct contact of the LED chip with the heat dissipating element 10 and its curved indentations 101.
In U.S. Pat. No. 6,705,393 B1, TW Patent No. 213446 and No. 220351, a ceramic structure with small pores is proposed, so as to increase heat convection for heat dissipation. However, the objective of this patent is to provide a ceramic heat dissipating element that dissipates heat during operation of a CPU of a computer. Nothing further is described on how the heat dissipating design is to be applied to LED packages.
From the above, it is clear that current high-power LED packages only concentrates on increasing the heat conduction from the junction of the LED chip to external of the package. Nonetheless, the heat in the whole system eventually has to be completely removed from the LED to outside of the system via air convection. Moreover, in the conventional designs using heat conduction and convection for heat dissipation, larger elements or other materials with high thermal conductivity need to be added, which occupies more space and impedes smaller-size applications.
Therefore, there is a need for high-power LEDs with high thermal conductivity and heat convection.